This invention relates to a sheet feeding cassette latching system suitable for use in latching to a main body of a copying apparatus, for example, a sheet feeding cassette comprising a sheet support plate supporting a stack of sheets and pivotably held at its rearward end by a bottom plate of the cassette which is forced by a push-arm secured to a rotary shaft of the main body of the copying apparatus into pressing engagement with the uppermost sheet of the stack of sheets on the sheet support plate to feed one sheet after another to a copying station.
A sheet feeding cassette is usually used with an electrophotographic copying apparatus, printer, facsimile system, etc., as a device for feeding sheets, such as transfer-printing sheets, copy sheets, etc. Generally, such sheet feeding cassette which comprises a sheet support plate supporting a stack of sheets thereon and pivotably supported at its rearward end by a bottom plate of the cassette is latched on to a main body of the copying apparatus, for example, and the sheet support plate is forced to move upwardly by a pushup member of the copying apparatus to bring the uppermost sheet of the stack of sheets on the sheet support plate into pressing engagement with a pickup roller for picking up one sheet after another to feed same to a copying station. In this type of sheet feeding cassette, the sheet support plate is moved upwardly by applying pressure thereto by forcing the pushup member against the under-surface of the sheet support plate by the biasing force of a tension spring. When the sheet feeding cassette of this construction is latched on to the copying apparatus main body or unlatched, it is necessary to actuate a change-over lever or a cam located at one side of the cassette to temporarily move the pushup member away from the under-surface of the sheet support plate. Because of this, the sheet feeding cassette of the prior art has suffered the disadvantage that a shock of high magnitude and a noise of high level are produced when the cassette is introduced into the main body of the copying apparatus and the pushup member flips the sheet support plate of the cassette upwardly. In the type of cassette in which the pressure is released by means of the change-over lever, the operation is troublesome while a force of high magnitude is required for operating the lever and there is the disadvantage of the cassette or the main body of the copying apparatus suffering damage if an attempt is made to perform cassette latching or unlatching operation without releasing the pressure. In the type of cassette wherein the cam located at one side of the cassette is relied on, a force of considerably high magnitude is required for performing the cassette latching and unlatching operations, so that this type of cassette is lower in operability than the previously mentioned type of cassette. A sheet feeding pressure is produced as the load of the tension spring is applied between the sheets and the sheet feeding means. This pressure may vary depending on the displacement of the spring or the weight of the sheets, so that there is the disadvantage that a stable sheet feeding pressure is unobtainable depending on the volume of the stack of sheets on the sheet support plate or the size of the sheets.
When the cassette is replenished with sheets or the sheets in the cassette are replaced by sheets of different size or quality, it has hitherto been usual practice to unlatch the sheet feeding cassette from the main body of the copying apparatus. Thus, the sheet feeding cassettes of the prior art have generally been low in operability. To cope with this situation, a proposal has been made to use a sheet feeding cassette which comprises an upper cover that can be opened or closed while the cassette remains latched to the main body of the copying apparatus, and a sheet support plate pushup member which is operable in conjuction with the opening and closing operations of the upper cover to be separated from the sheet support plate to allow same to move downwardly to thereby provide a clearance between the sheet feeding means and the sheet support plate or the uppermost sheet of the stack of sheets placed thereon, so that sheets can be inserted through the clearance below the upper cover. In this case, a sheet feeding pressure is applied by means of a tension spring, so that a force of considerably high magnitude is required to perform upper cover opening and closing operations and cassette latching and unlatching operations. Also, a variation in the sheet feeding pressure is inevitable depending on the volume and size of the sheets placed on the sheet support plate.